The ability to locate persons, vehicles, and the like has become more important in recent years, particularly in view of new technologies being introduced and increasing concerns over safety and security. A person or vehicle can be located by determining the location of a mobile radio device carried by the person or vehicle. For example, it is desirable to provide a cellular telephone system with the ability to determine the geographic location of an individual cell phone used to report an emergency so that such location can be reported to emergency response workers such as police, fire and ambulance services.
Several schemes for determining one's position on Earth are available. One way to determine position involves the use of the global positioning system (GPS). The GPS was originally conceived and developed by the U.S. Department of Defense as a military navigation system. Over time, elements of the system have become increasingly available for civilian use. The GPS uses a constellation of 24 satellites, in a geo-stationary orbit, whereby position can be determined by timing satellite signal journeys from a GPS satellite to a GPS receiver. Five spare orbiting satellites are provided primarily for backup in case one of the 24 satellites fail. The satellites transmit spread-spectrum signals on two frequency bands L1 (1575.42 MHz) and L2 (1223.6 MHz). The signals are modulated by two pseudo-random noise codes; a coarse/acquisition (C/A) code and a precision (P) code. The C/A code in the L1 band is the code pertinent to civilian applications. Additionally, the GPS signal is modulated with a data message commonly referred to as the GPS navigation message.
Typically, a GPS receiver employs a trilateration scheme in an effort to obtain a position fix. For instance, a GPS-derived position can be accomplished using two-dimensional trilateration. For example, signals from three satellites can be used to determine position based on the intersection of three intersecting circles. More specifically, each satellite signal can provide a radius in which the GPS receiver can lie. Two intersecting radii allow the position determination to be narrowed to the area of intersection. Another satellite signal can provide a third radius indicative of the position of the GPS receiver since all three radii should intersect at a single point. Expanding the forgoing concept to three-dimensional trilateration, each satellite signal can be used to indicate a sphere whereby three intersecting spheres can used to determine position which includes altitude information. More satellite signals can be used, and typically are used to improve accuracy.
At the GPS receiver, the satellite signal is demodulated after it is matched and synchronized with a pseudo-random noise code. The GPS receiver uses the GPS navigation message to calculate satellite signal transit times in addition to the coordinates of the GPS satellite. Position measurement by a GPS receiver can typically be accomplished within 15 meters (50 feet). However, the accuracy of these calculations are dependent upon measurement accuracy and satellite configuration. Atmospheric conditions can cause ionospheric delays. Additionally, uncertainties in satellite orbits can contribute to errors since, axiomatically, satellite orbits degrade over time. Reliance upon position indication using GPS data can be additionally problematic in view of public safety concerns in today's environment.
In order to address signal availability problems associated with the GPS, server-assisted GPS was introduced in the late 1990s. Stationary server computers are provided with a stationary GPS receiver for receiving GPS satellite signals. The stationary GPS receivers are associated with an antenna having a full view of the sky in order to allow continuous monitoring of the signals from all visible GPS satellites. A radio interface is provided with each server to allow communication with mobile GPS stations. In connection with a position query about a mobile GPS unit's position, the server transmits its GPS satellite information, obtained from its stationary GPS receiver, to the mobile GPS unit. This information includes a list of observable GPS satellites and data which allow the mobile GPS receiver to synchronize and match psuedo-random noise codes with those codes of the GPS satellites. The mobile GPS receiver transmits its collected GPS data to the server. The server, in turn, computes the position of the mobile GPS receiver from the data provided by the mobile GPS and the stationary GPS. While this scheme permits greater accuracy over non server-assisted GPS, satellite signal availability can still cause problems in acquiring an accurate position.
The enhanced signal strength (ESS) system employs a position location scheme which is independent of the GPS. Three-dimensional information covering the terrain including buildings, structures and other obstructions is collected in order to model radio frequency signal propagation characteristics for a wireless transmitting antenna in a given geographic area of interest. The results of the modeling are stored in a database. The position of a mobile locator is determined in connection with the locator measuring the signal strength of a signal from a number of wireless transmitters. The position is calculated by the system using input information from the mobile locator and the stored database information. This system has been used in Japan in connection with the Personal Handy Phone System (PHS).
Other schemes for determining position, without using the GPS, make use of either the angle of arrival (AOA) of signals at receivers or the time difference of arrival (TDOA) of signals at receivers.
The network-based angle of arrival scheme determines the location of a mobile station (as, for example, a mobile phone, a personal digital assistant with wireless communications capability, a portable computer with wireless communications capability, a pager or other personal communications device) by determining the angle from which a signal is arriving at two or more fixed antenna sites. For instance, signal direction or angle of arrival at each site can be determined from the difference in time of arrival of incoming signals at different elements of a single fixed antenna at that site. For instance, a two element phased array antenna can be used to cover angles between 60° and −60°. A six element phased array antenna, which is equivalent to three antennas with 2 pairs of elements, can cover 360°. Equipment within the communication network combines the angle data from multiple sites to determine the location of the mobile station. Proper angle measurement and the geometric relationship between the mobile station and fixed antennas can affect position measurement. Proximity of the mobile station to the mid point between two fixed antennas can cause significant position measurement error. For this reason, it is desirable to use three or more antenna sites in making AOA measurements.
The time difference of arrival scheme of determining position is another network-based solution which measures the time difference of arrival of a radio signal to at least two antenna sites. Using the speed of an electromagnetic wave and known transmit and receive times, the distance between a fixed antenna and mobile station can be determined. The processed information is translated into longitude and latitude position readings. The accuracy of synchronized clocking information necessary to properly compute TDOA is critical to proper position measurement. Synchronized accurate clocking can sometime be problematic in TDOA measurement. TDOA position measurements can suffer as a consequence thereof. A mere micro second clocking error can contribute to several meters of error in position measurement.
Forward link trilateration can also be employed to determine position whereby the time difference of signal arrival from a base station antenna to a mobile station can be calculated by measuring the phase difference between pseudo-random noise coded signals being transmitted from at least two antennas to the mobile station. This scheme is particularly useful for code division multiple access (CDMA) systems. Advanced forward link trilateration (AFLT) is a variation of this scheme wherein the mobile station and base stations reverse roles. In AFLT, the position of the mobile station is fixed in connection with the base stations receiving transmissions from the mobile station. In AFLT, the mobile station measures CDMA phase offsets of different pilot phase noises and reports them to the position determination entity at the network. The position determination entity uses the different pilot phase measurements to perform forward link trilateration to compute the position fix for the reporting entity.
Fingerprinting provides another approach to determining the position of a mobile station. Radio frequency signal characteristics associated with various regions in a signal transmission area are collected in a database. Each grouping of signal characteristics for a region is known as a fingerprint. The position of a mobile station is determined by comparing an RF data sample collected by the mobile station to fingerprint data in the database. The comparison can be made at the mobile station or at the server holding the fingerprint data. The fingerprint data collected benefits from the collection of multi-path signals which arise through indirect signal paths from transmitter to receiver. While not subject to many of the problems associated with other position identifying technologies, fingerprinting requires substantial work in data collection and is therefore economically feasible only for highly populated, highly concentrated metropolitan areas.
Thus, as described above, numerous individual position-determining schemes are known in the art. These schemes can be categorized broadly as mobile station assisted modes (MS-Assisted mode) and mobile station based/standalone mode (MS-Based/Standalone modes). In the MS-Assisted mode, the position of the mobile station is determined by the by a computer commonly referred to as the Network/position Determination Entity (PDE), which computer is connected to the communications network. The PDE can employ one of the methodologies outlined above as, for example, TDOA, AOA, ESS, etc. In MS-Based/Standalone mode, the mobile station computes its own position location using its processor using data available at the mobile station. One example of an MS-Based/Standalone system is a system in which the mobile station is equipped with GPS receiving and processing capability, and determines its position based on GPS signals received at the mobile station. Individual scheme of position determination depends upon different elements/resources which are susceptible to different types of errors. For instance, network connection is needed in all GPS MS-Assisted methods regardless of the scheme (GPS, GPS plus AFLT, etc.) chosen. Even if the GPS measurements are excellent, the MS-Assisted method is going to fail if the network connection fails. Therefore in this instance, a stand-alone GPS methodology would provide a better location estimate.
Each of the foregoing schemes for determining position may be inaccurate or unavailable on occasion as noted above. Thus, a mobile station or network using any of these known position-determining schemes may fail to obtain any result when asked to determine the current position of the mobile station or may obtain an inaccurate result. A need therefore exists to create a method for more reliably determining the position of a mobile station with greater confidence.